1. Field of the Invention
The present invention relates to a liquid crystal display device, referred to hereinafter as an LCD, and particularly to a structure by employing plural micro domains respectively having different pre-tilt angles for enhancing the viewing angle range of a vertical alignment type LCD.
2. Description of the Related Arts
There have been attempted many kinds of improvements to enhance the range of viewing angle looking at LCD panels. As one of them, there is a method employing a vertical alignment type liquid crystal. There is another method that a single picture cell, referred to hereinafter as a cell, is divided into plural domains, which may be generally called micro domains, having different viewing angle characteristics so that the average of these characteristics is used to enhance the viewing angle characteristics. Furthermore, the viewing angle can be further extended by combining these two techniques.
The prior arts related to these techniques are hereinafter described with reference to FIGS. 14 to 17. FIG. 14 is of the case where the technique called "surrounding electrode method" is applied to a vertical alignment type LCD; and the method is disclosed in detail in Japanese Unexamined Provisional Patent Publication Tokukai Hei 7-13164. Outside glass substrates 12 & 14 are provided with polarizing plates which are not shown in the figures. Upon lower glass substrate 12 are arranged display electrodes 32 and alignment control electrodes 34. Upon upper glass substrate 14 are arranged opposing electrodes 36 each having a slit called alignment control window 36 H at the center of the cell. Upon electrodes 32 & 36 are arranged vertical alignment films 51 & 52 , respectively. Between vertical alignment films 51 and 52 are sealed liquid molecules 50 having negative dielectric anisotropy. FIG. 14 illustrates the state of liquid crystal molecules 50 when the voltages are applied to the electrodes, which is hereinafter called "a voltage applied state" or an ON state.
When no voltage is applied to the electrodes, which is hereinafter called "a no-voltage applied state" or an OFF state, all the liquid crystal molecules are aligned almost vertical such as molecule 54 shown in FIG. 14. However, on the ON state molecules 53 excluding molecules 54 align not so straight as shown in FIG. 14. In controlling the alignment state, alignment control electrode 34 and alignment control window 36 play an important roll. At first, the alignment control electrode 34 acts to cause the electric field to bend as shown with the numeral 55. Consequently, the electric fields in the cell can be tilted outwardly as denoted with the numeral 56. Next, the electric field at the center of the cell is dispersed by forming alignment control window 36H at the central portion of opposing electrode 36 so that a region having a weak electric field can be formed at the center. Thus, the functions of alignment control electrodes 34 and alignment control window 35H can provide the liquid crystal molecules 53 & 54 with two kinds of alignment directions.
The liquid crystal display device shown in FIG. 14 employs for the alignment film a vertical alignment film having a uniform characteristic throughout the cell. On the contrary, there is a liquid crystal display device having an alignment film having in a single cell a plurality of domains respectively having different alignment characteristics as shown in FIGS. 15 to 17. The details are disclosed in Japanese Unexamined Patent Publication Toku Kai Hei 7-281176.
The latter liquid crystal display device shown in FIGS. 15 to 17 is constituted with a pair of first and second glass substrates 12 & 14 having liquid crystal 40 sealed therein, and polarizing plates 16 & 18 arranged respectively upon the outer sides thereof. Upon first glass substrate 12 are provided a color filter 20, a first transparent electrode 22 and a vertical alignment film 44. Upon second glass substrate 14 are provided a second transparent electrode 26 and a vertical alignment film 48.
Liquid crystal 40 is of a negative dielectric anisotropy, and is added with a chiral-dopant to accelerate the twist of the liquid crystal. Accordingly, when the voltage is not applied thereto the liquid crystal molecules are aligned almost vertical to the glass substrate surface as shown in FIG. 17A; and when the voltage is applied thereto the liquid crystal molecules are tilted towards the glass substrate surface so as to twist according to respective rubbing directions of the paired substrates and helical power of the liquid crystal itself as shown in FIG. 17B.
Here, vertical alignment films 44 & 48 have been treated so as to be divided into respective domains A & B as shown in FIGS. 15 & 16. In domain A, a first vertical alignment film 44 has been treated, i.e. rubbed, so that the liquid crystal molecules which contacts first vertical alignment film 44 have a pre-tilt angle .alpha. with respect to the substrate surface, while a second vertical alignment film 48 has been rubbed so that the liquid crystal molecules which contacts second vertical alignment film 48 have a pre-tilt angle .beta. with respect to the substrate surface. Pre-tilt angles .alpha. & .beta. are set close to 90.degree., however, are .alpha.&lt;.beta. by controlling the condition to form vertical alignment films 44 & 48. In the voltage application, liquid crystal molecules in between substrates 12 & 14 are likely to tilt according to the smaller pre-tilt angle a as shown in FIG. 17B. Then, pre-tilt angle .alpha. is, for example, 85.degree.; and pre-tilt angle .beta. is, for example, 89.degree..
On the contrary, in the adjacent domain B, second vertical alignment film 48 has been rubbed so that the liquid crystal molecules which contacts first vertical alignment film 44 have a pre-tilt angle .alpha. with respect to the substrate surface, while first vertical alignment film 44 opposing thereto has been rubbed so that the liquid crystal molecules which contacts second vertical alignment film 48 have a pre-tilt angle .beta. with respect to the substrate surface, where also pre-tilt angles .alpha. & .beta. are set close to 90.degree., however, are also set so that .alpha.&lt;.beta.. On the voltage application, the liquid crystal molecules at the mid of paired glass substrates 12 & 14 are likely to tilt following pre-tilt angle, .alpha., the smaller angle.
The region to correspond to a single cell becomes to have a viewing angle characteristics which is an average of viewing angles of two domains A & B because the region corresponding to the single cell is divided into two domains A & B respectively having viewing angle characteristics different from each other by 180.degree.. Accordingly, this liquid crystal device can be provided with a largely extended viewing angle range.
The prior art illustrated in FIG. 15 is of a case where the twist angle in the voltage application is 45.degree..
In comparing the above two kinds of liquid crystal device, the former liquid crystal device shown in FIG. 14 has one more constitutional element than the latter liquid crystal device shown in FIGS. 15 to 17. That is, alignment control electrode 34 is newly required, which causes not only more complex structure but also a disadvantage in its aperture characteristic.
In the structure of the latter liquid crystal device shown in FIGS. 15 to 17, there may take place a case where the domain formation becomes unstable on the voltage ON state. More specifically, liquid crystal molecules 40C to be aligned almost vertically at the boundary of the domains of FIG. 17 do not always stably exist at the boundary of domains A and. B, but may be formed at a place deviated from the boundary, or may move around the boundary. In these cases, the balance of the area ratio of two domains A & B is deteriorated; accordingly, there is a problem in that not only the viewing angle characteristic is deteriorated but also the expansion of the discontinuous portion at the domain boundary further deteriorates the viewing angle characteristics as well as the aperture.
Therefore, in the latter liquid crystal device it is important to stably form the discontinuous portion at the boundary of two domains A and B.